1. Field of the Invention
This invention relates to an image projector apparatus for producing an image for display and, more particularly, to such apparatus in which a light beam from a light source is transmitted through an image medium so as to provide the light with image information and project the light therefrom.
2. Description of the Prior Art
In the prior art, transmission light adjusting image projectors may increase the size of a picture so as to enable a number of people to view the picture. An example of such a projector is illustrated in FIG. 10. As shown therein, a projector 45 generally includes a light source 10, a liquid crystal display (LCD) 11, a projection lens 12, a signal processing device 43, and an optical control circuit 44.
In the projector 45, a light beam is radiated from the light source 10 to a plurality of pixels (not shown) contained in the LCD 11. An image signal is supplied from an input terminal 42 to the signal processing device 43, whereupon the image signal is processed in a predetermined manner and supplied therefrom to the optical control circuit 44 which, in turn, generates an optical control signal in accordance with the processed image signal. The optical control signal is supplied to the LCD 11 so as to control the opening and closing of a shutter of the LCD, thereby controlling the transmittance of each pixel of the LCD. As a result, an image may be projected from the LCD 11 through the projection lens 12 onto a projection screen 13.
In the projector 45, the light source 10 normally radiates a relatively high intensity level of light to compensate for the reduction of luminance caused by the relatively high expansion or enlargement of an image. Further, each of the pixels in the LCD 11 has a relatively small opening corresponding thereto for enabling light transmission. As a result, the luminance level of the image radiated from the LCD and projected onto the projection screen 13 is relatively low. Furthermore, the operating life of a lamp typically used as the light source 10 is relatively short and the power consumption is relatively high.
Thus, it is desirable to effectively utilize the radiated light from the light source 10 and to increase the efficiency of the radiated light transmitted through the LCD 11 so as to eliminate or minimize the amount in which the luminance level is lowered. Such increased efficiency may enable a lamp which provides a lower intensity level of light, and which has a lower power consumption and dissipates a corresponding lesser amount of heat, to be used.
The sizes of the open portions of the LCD 11, or opening ratio, which respectively correspond to the pixels, are controlled by a shielding mask (not shown), which may be arranged on the incident side of the LCD. By decreasing the area of the shielding mask, the size of the open portions of the LCD may be increased as viewed from the incident surface thereof.
Due to the shielding mask and the relatively small sized open portions of the LCD 11, and when the distance from the screen 13 is increased and the picture is enlarged, the image projected on the projection screen may appear grainy, thereby resulting in an image or picture having relatively poor quality.
Further, as shown in FIG. 10, the distance between the projection lens 12 and the projection screen 13 is represented by "a", while the distance between the LCD 11 and the projection lens is represented by "b". Accordingly, the focal length of the projection lens 12 may be determined by the following equation: EQU (1/a)+(1/b)=(1/f) (1)
As a result of the distances a and b, the image projected on the projected screen 13 is approximately b/a times larger than the image on the LCD panel. As is apparent from equation 1, the focus of the image is dependent upon the distance from the projection screen 13.
The projector 45 may be utilized in an airplane or the like. In such situation, the projector 45 is suspended from the ceiling so as not to disturb the passengers and the projection screen 13 is positioned so as to be visible by the passengers. Due to such positioning of the projector 45 and the projection screen 13, a center axis of the projector is not perpendicular to the projection screen. As a result, a so-called keystone distortion may be produced. FIG. 11 illustrates such arrangement between the projector 45 and the projection screen 13.
As an example of such keystone distortion, consider the situation in which an image of a square object is radiated from the projector 45 for display on the projection screen 13, in which the projector and projection screen are mounted in an airplane in the manner previously described. In this situation, instead of a square shape, the object displayed on the projection screen 13 has a trapezoidal shape.
Such keystone distortion may be reduced by using a projection lens 12 having a relatively large diameter. However, using such large diameter projection lens has several disadvantages. That is, such projection lens may cause the image light radiated therefrom to have a relatively large diameter, thereby lowering the light volume or luminarice level of the image projected on the projection screen 13. Further, as is to be appreciated, the use of such large diameter projection lens may increase the size and cost of the projector 45.
The size of the projector 45 is normally relatively large. That is, as previously described, the projection lens used to reduce keystone distortion is relatively large and, as such, causes the projector to be relatively large. However, even the "other" projection lens has a large diameter which still causes the projector to be relatively large. Further, due to the distance b normally required between the LCD 11 and the projection lens 12, the projector 45 typically has a relatively large length or depth. Accordingly, due to the diameter of the projection lens and the distance b, it is difficult to construct the projector 45 so as to have a relatively small size.
Thus, the prior art has failed to provide a relatively small-sized and low cost projector for producing an image for display on a projection screen having relatively good picture quality, which is capable of correcting keystone distortion and adjusting the focus.